On Sep. 6, 1960, Eugene Stoner was granted U.S. Pat. No. 2,951,424 for a “Gas Operated Bolt and Carrier System” which is used in the design of the AR-10, AR-15, M-16 and M-4 series of rifles (sometimes referred to herein collectively as “AR-15 rifles” or “AR-15's”). This system is referred to as a “direct impingement system” because some of the gas from a fired cartridge is redirected so that it interacts directly with the rifle's bolt carrier assembly to actuate it. Specifically, as it relates to the rifles listed above, the gas drives the bolt carrier assembly backward, ejecting the shell. A recoil spring then pushes the bolt carrier forward again allowing a new cartridge from a magazine to be loaded into the barrel, thus completing the cycle of the bolt carrier assembly, wherein the bolt carrier moves back to its original position. Despite some drawbacks, the Stoner design has been extremely successful as proven by its use in millions of rifles for many decades.
The main advantages of the Stoner design relate to the simplicity of the gas system and the fact that all moving parts of the recoil systems are in line with the bore. Specifically, the muzzle, barrel, bolt, bolt carrier, buffer and recoil spring all exist along the same axis in the Stoner design giving these rifles low perceived recoil and improving accuracy by limiting muzzle rise. However, a disadvantage of the Stoner system is that the recoil buffer and recoil spring are typically located in the stock of the rifle limiting certain modifications to the rifle which can be made to those using different types of recoil systems. For example, until recently, this design did not allow for the use of a folding stock, which can be very useful for rifles used in vehicles, planes and other places where space is limited. This limitation was overcome by U.S. Pat. No. 8,769,855 which disclosed a folding stock adapter for use with the AR-10, AR-15, M-16 and M-4 series of rifles. However, even this system does not allow a rifle to be fired repeatedly when the stock is in a folded position.
The later generation AR-18 was developed using the same rotating-bolt locking mechanism used in the Stoner design, but also used a shorter recoil system comprising two short recoil springs on guide rods rather than one large recoil spring located within the stock as found in the AR-15. The AR-18 uses a piston system rather than a direct impingement system, meaning that the gas actuates a piston, rather than directly actuating the bolt carrier system such as the SDIGS, which then actuates the bolt carrier system. The shorter recoil system of the AR-18 is located in the upper receiver, taking any function of the stock out of the recoil system, thus allowing the stock to be folded or even removed without affecting the weapon's ability to function properly. However, a drawback of the recoil system used in the AR-18 is that the gas piston system is located in the upper receiver, which is not in line with the barrel of the rifle. Rather, the recoil system of the AR-18 is in line with the gas piston system and operating rod, placing its recoil springs above the bolt carrier and above the barrel and bore. Because the operating rod and recoil system is above, and not in line with the muzzle, barrel, and bolt, a loss of accuracy due to muzzle rise and perceived recoil is possible.
What is needed is a shortened recoil system that uses direct impingement and is in line with the barrel and bore of the rifle.